The invention relates to an electric furnace. Such a furnace is known for example from the French Patent Specification No. 382,457.
Advances in the development of semiconductor components in previous years have brought about increasing usage of direct current arc furnaces in the iron and steel industry for the smelting especially of electric steel.
The construction and method of operation of direct current arc furnaces are known for example from the journal "Stahl und Eisen" ("Steel and Iron"), 103 (1983) No. 3, 14th Feb. 1983, pages 133 to 137.
In the case of direct current arc furnaces, in order to optimise the electrical and thermal relationships, it has been shown to be advantageous to form the arc between one or more electrodes, arranged above the melting charge, and the melting charge itself. At least one electrode, the bottom electrode, which is in the bottom of the furnace and is in contact with the melt, is provided for the return circuit of the direct current. The bottom electrode is exposed to a continuous and very high thermal stress, for which materials having a very high fusion and melting point, for example graphite, are suitable. On the one hand, however, when carbon electrodes are used the melt is carbonised. But this is not desirable, especially during the manufacture of low-carbon steels. On the other hand, the carbon electrodes are consumed, which can weaken the furnace bottom and unfavourably affect the electrical power transmission.
The furnace according to French Patent Specification No. 382,457 consists of a combined Siemens-Martin and electric arc furnace, by which means the advantage of the Siemens-Martin furnace on the one hand--the possibility of carrying out metallurgical slag smelting--and the advantage of the electric arc furnace on the other hand--to overheat the melting bath and carry out metallurgical refining process--can be utilised simultaneously.
Moreover, several bottom electrodes are arranged in a curved furnace bottom extending in the longitudinal direction. A bath movement is brought about by the electromagnetic field of the current flowing through the melting bath from the bottom electrode to the top electrode, which bath movement is particularly intense at the melting bath contact surfaces of the bottom electrodes, where there is a marked change in the strength of the electromagnetic field, that is, at those transition points where the electric current passes over from the relatively small cross section of the bottom electrode to the relatively large cross section of the melting bath.
The melting bath flow acts on the melting bath contact surfaces which now melt back under the effect of temperature slightly behind the hearth surface, which causes small indentations, so called craters, to form. As a consequence of the relatively high kinetic energy of the bath flow, a cross flow (secondary flow) is induced in these indentations. This causes the contact surfaces to melt down still further. However, melting down of the contact surfaces of the bottom electrode at its end facing towards the melting bath is to be avoided if possible or at least reduced to a harmless level, because of craters (local cavitation) are not only restricted to the contact surfaces but also affect the adjacent areas of the refractory structural material, so that crater-like recesses develop. When the liquid charge is poured out of the furnace, the craters are then likewise emptied and hollow spaces develop which impede subsequent electrical contact of solid constituents to be melted.
The intensity of the bath movement is of course also dependent on the strength of the electromagnetic field. For a predetermined current intensity, this electromagnetic field becomes weaker the longer the magentic field lines are, that is, the greater the periphery of diameter of the bottom electrode is.
Because the forces directed towards the melting bath act at right angles to the electromagnetic field lines, a bath movement forms which is directed at right angles towards the magnetic field lines, that is, from outside towards the axis of the bottom electrode.